Very high power (kW) lasers are used in a wide variety of applications. Presently, the maximum achievable powers are .about.300 W average. However, the lasers used are operated in a pulsed mode to avoid overheating and related problems; hence are only quasi-Continuous Wave. The power limitation is due to inherent difficulties in producing high gain media, including heat dissipation from the active rod, damage threshold of the elements, durability, power consumption, and cost. Due to the nonlinear nature of lasers, these issues are far less significant in low power lasers. An ideal solution for very high power lasers is to combine outputs of several lasers with lower power. This beam combining could be done intra-cavity or externally depending on the application. Combining broad band, polarization independent light from several incoherent light sources are of special interest for projection displays. This would allow for brighter output from several smaller light sources. In addition, by using polarizers and wave-plates followed by recombination, an unpolarized beam can be turned into a polarized beam, effectively without loss.
There have been many attempts to combine laser beams, using both linear and nonlinear optics, with little or no success. The methods used include (i) focusing two beams into a single fiber and (ii) use of photorefractive beam coupling. The former results in significant loss and is not suitable for high power lasers due to damage to the fiber. The latter requires both beams to be coherent with respect to each other; therefore, they must originate from the same laser. People familiar with the art know that the output beams from two lasers are not coherent with respect to one another and therefore, cannot be combined in a conventional photorefractive geometry. Both methods are wavelength dependent and cannot be used with a broadband source. To date, there are no known effective beam combiners in any laser system.
Conversion of unpolarized into polarized light sources has been the subject of several investigations. These methods involve the use of reflecting polarizers followed by some conversion mechanism. None of these systems, however, have proved useful due to their inefficient performance. Furthermore, these systems cannot be used to combine light from several different light sources. There is considerable interest, in combining polarized or unpolarized light depending on the required performance.
The primary obstacle to certain strategies of beam combining are physical constraints originating from energy conservation.
Origin of the Problem
The fundamental difficulty in combining beams can be illustrated by considering the special case of combining two plane polarized, monochromatic coherent beams of the form EQU E.sub.1 =e.sub.1 E.sub.o f.sub.1 (x,y) cos (k.sub.1.multidot.r-.omega.t) (1) EQU E.sub.2 =e.sub.2 E.sub.o f.sub.2 (x,y) cos (k.sub.2.multidot.r-.omega.t+.phi.) (2)
where the E is the electric field, k is the wavevector, .omega. is the angular frequency and .phi. is the phase difference between the two beams. The polarization direction is given by e; the transverse beam profiles are given by f.sub.1 (x,y) and f.sub.2 (x,y). The photon flux (the number of photons crossing a surface per unit time) for each beam in a material with dielectric constant .di-elect cons. is proportional to the energy flux, J, which to a good approximation is given by: ##EQU1##
where dA is an element of area and c is the speed of light. The total flux for both beams is ##EQU2##
If the beams were combined, the flux for the two collinearly propagating beams is: ##EQU3##
In the steady state, in a passive nonabsorbing device, the total flux is conserved. Comparing Eqs. 5 and 6, energy conservation requires EQU (e.sub.1.multidot.e.sub.2,.intg. cos (.DELTA.k.multidot.r+.phi.)f.sub.1 (x,y)f.sub.2 (x,y)dA=0 (7)
Since we are interested in combining collinearly propagating, monochromatic light (i.e., .DELTA.k=0), the condition reduces to: EQU cos (.phi.)(e.sub.1.multidot.e.sub.2).intg.f.sub.1 (x,y)f.sub.2 (x,y)dA=0 (8)
Any successful beam combiner must satisfy this condition. In particular, the optical element must be constructed so that either, (i) the relative phase between the two input beams is exactly 90.degree., or (ii) they have orthogonal polarization, or (iii) the overlap integral vanishes.
The above requirement is for monochromatic sources, and holds when beams from a number of identical sources are combined. It does not apply only if the beams to be combined have different wavelengths.
Beams With 90 Degrees Phase Difference (cos(.phi.)=0)
If .phi.=90.degree., cos.phi.=0 and Eq. 7 is satisfied. This method has been demonstrated in photorefractive beam coupling experiments. In these experiments, two coherent beams are crossed in a photorefractive medium. The interference pattern set up by the two beams generates a grating of charge carriers. Due to the internal field, the carriers move to the dark regions of the interference pattern and set up a space charge field with the vacancies that are left behind. This space charge field alters the refractive index through the linear electro-optic effect. The index change has the periodicity of the initial interference pattern but is offset by a phase difference of 90 degrees. Energy can therefore be transferred from one beam to another without violating energy conservation. Moreover, there is evidence of energy transfer from one beam to another in certain materials. However, the beams used must originate from the same laser and be coherent for the interference pattern to be realized. Photorefractive materials therefore cannot be used to combine beams from different laser sources since these are not coherent with respect to one another. Furthermore, their efficiency is wavelength and temperature dependent, and they typically cannot operate much beyond near IR where most common high power lasers operate.
Alternatively, one can combine two beams via a beam splitter, such as a half-silvered mirror oriented at 45.degree. to the direction of propagation, as in the output of a Mach-Zehnder interferometer. The beams transmitted and reflected by the beam splitter will be out of phase by 90 degrees. Consequently, two beams can be combined so that they will propagate in a given direction if (and only if) the two beams are coherent. As in the case of the photorefractive effect, this method can also not be used to combine beams from two different lasers.
Use of Orthogonal Polarizations (e.sub.1.multidot.e.sub.2)=0
The vanishing of e.sub.1.multidot.e.sub.2 allows the combination of two orthogonally polarized beams. A polarizing beam splitter is an element which utilizes total internal reflection within a birefringent material to separate the components with different polarizations into two orthogonally propagating beams. Using this, one can clearly bring two beams of orthogonal polarizations together and combine them into a single collinear beam. However, this can only be done once. In other words, this technique can only be applied for two beams of orthogonal polarizations. The combined beam will have a random polarization if the two beams are not coherent. Consequently, no additional beams from other lasers can be added. It should also be noted that even for combining two beams, it is necessary for the output from the lasers to be polarized.
Forcing the Overlap Integral to Vanish
To combine beams from a number of lasers, therefore, the only scenario allowed by energy conservation is forcing the overlap integral to vanish. This method allows the combination of incoherent beams regardless of polarization, and can, therefore, be extended to several laser beams. In this case, it is required that the integral EQU .intg.f.sub.1 (x,y)f.sub.2 (x,y)dxdy=0 (9)
This is achieved if the beams do not overlap. Simply arranging beams "side-by-side" is an example of this case. However, this will severely alter the profile of the combined beam and is therefore not useful in practice.
Therefore, there is a need for a low cost optical element and method for use which can combine several beams with little alteration of their characteristics. The element should be constructed using either linear or nonlinear optical materials to allow for usage ranging from projection systems to optical computing. Furthermore, there is a need for a system that can be used to combine both coherent and incoherent beams and is wavelength and polarization independent over a large spectral range. Due to its geometry, the needed element will need to operate below the damage threshold if the input beams used are from high power lasers. There is also a need for a stand-alone system to combine beams external to a laser cavity or which can be placed within the cavity for coherent combining. There is also a need for an element which can be implemented in conjunction with other methods, such as those utilizing the photorefractive effect, to allow for high efficiency in those techniques.